In a swash plate type piston pump motor, when a piston reciprocates within a cylinder bore by rotation of the cylinder block, a very high contact pressure is generated at a contact surface between the piston and the cylinder bore. Then, in a case where the swash plate type piston pump motor is used for construction machinery or the like under high pressure or in a case where working fluid is fire resistant fluid, fire resistant refrigerant, or the like and has low lubricity, seizure may be generated at the piston or the cylinder bore.
In such a background, the swash plate type piston pump motor described in Japanese Patent No. 4828371 (Patent Document 1) has been proposed as the swash plate type piston pump motor capable of suppressing the seizure. This swash plate type piston pump motor includes a cylinder block and a piston which is slide-fitted (inserted) into a piston chamber of the cylinder block so as to be capable of advancing and retreating. A taper part whose diameter is reduced from a piston inserting side to another side is formed on an inner circumferential wall of the piston chamber. Further, a tilt angle of the taper part is set to from 1° to 10°. Patent Document 1 describes that since the tilt angle of the taper part is set to from 1° to 10°, a contact stress between the piston and the cylinder can be reduced.